Pretreatment Method for Reduction and/or Elimination of Basal Plane Dislocations Close to Epilayer/Substrate Interface in Growth of SiC Epitaxial Films

ABSTRACT

Non-destructive pretreatment methods are generally provided for a surface of a SiC substrate with substantially no degradation of surface morphology thereon. In one particular embodiment, a molten mixture (e.g., including KOH and a buffering agent) is applied directly onto the surface of the SiC substrate to form a treated surface thereon. An epitaxial film (e.g., SiC) can then be grown on the treated surface to achieve very high (e.g., up to and including 100%) BPD to TED conversion rate close to the epilayer/substrate interface.

PRIORITY INFORMATION

The present application claims priority to, and is a continuation of,U.S. patent application Ser. No. 13/682,240 titled “Pretreatment Methodfor Reduction and/or Elimination of Basal Plane Dislocations Close toEpilayer/Substrate Interface in Growth of SiC Epitaxial Films” ofSudarshan, at al. filed on Nov. 20, 2012; and claims priority to U.S.Provisional Patent Application Ser. No. 61/716,020 titled “A Method forElimination of Basal Plane Dislocations and In-Grown Stacking Faultswith No Surface Degradation for High Quality SiC Epitaxial Films” ofSudarshan, et al. filed on Oct. 19, 2012; to U.S. Provisional PatentApplication Ser. No. 61/638,770 titled “Method of Growing High Quality,Thick Silicon Carbide Homoepitaxial Films by Eliminating Silicon GasPhase Nucleation and Silicon Parasitic Deposition” of Sudarshan, et al.filed on Apr. 26, 2012; and to U.S. Provisional Patent Application Ser.No. 61/563,250 titled “Substrate Pretreatment Method forReduction/Elimination of Basal Plane Dislocations and In-Grown StackingFaults with No Surface Degradation for High Quality SiC Epitaxial Films”of Sudarshan, et al. filed on Nov. 23, 2011. The disclosures of all ofthese priority applications are incorporated by reference herein.

GOVERNMENT SUPPORT CLAUSE

This invention was made with government support under N00014-10-1-0530awarded by the Office of Naval Research. The government has certainrights in the invention.

BACKGROUND OF THE INVENTION

In SiC bipolar devices, basal plane dislocations (BPDs) in the epitaxialregions generate Shockley stacking faults (SFs) under device currentstress, and increase the forward voltage drift. In SiC epitaxial growth,the BPDs mainly come from the substrate. About 70-90% of BPDs on thesubstrate are converted to benign threading edge dislocations (TEDs),but 10-30% of BPDs propagate from the substrate into the epitaxial layercausing a BPD density of 10²-10³ cm⁻² in the epilayer. For sufficientyield (up to 90%) of bipolar devices, without forward voltage driftreliability problems, it is essential that the density of theseperformance-limiting defects in the epitaxial layer be less than 10cm⁻².

It has also been reported that in SiC epitaxial growth, the conversionof BPDs occurs during the growth of ˜20 μm thick epilayer, whichindicates that there may be BPDs buried within the 20 μm epilayer eventhough no BPDs are observed on the top surface of over 20 μm thickepilayer. These buried BPDs may still be converted to Shockley SFs undercurrent stress, and these SFs will extend into the drift layers anddegrade the device performance. The deeper the depth from the epilayersurface that the BPDs are buried, the higher the current density that isrequired to convert them to SFs.

In order to reduce the BPD density in the epilayer (i.e., enhance BPD toTED conversion rate), one of the effective methods is etching thesubstrate in a molten pure KOH or regular KOH—NaOH eutectic solutionprior to the epitaxial growth. Both etching methods need to generateetch pits at the points of BPDs intersecting the substrate surface. BPDetch pits >10 μm in diameter are required to achieve BPD density <10cm⁻² on the epilayer surface. In addition, due to the aggressive etchingmethods by molten KOH or regular KOH—NaOH eutectic, surface damagecannot be avoided even in the defect free regions of the substrate. Allof the etch pits (including BPDs, TEDs and threading screw dislocations(TSDs)) and the surface damage on the substrate will be replicated tothe epilayer surface. Post-polishing process is mandatory in order toobtain a smooth surface for device fabrication on the resultingepilayer, which reduces the practicability of the substrate etchingmethod.

Therefore, conversion of BPD to TED near the epilayer/substrateinterface without degrading the surface morphology is an important needfor the reliability of SiC devices.

Another one of the greatest challenges of growing SiC epitaxial films byhigh temperature chemical vapor deposition (CVD) is to restrict the gasphase nucleation or cluster formation or aerosol formation of siliconduring growth. These particles adversely influence the growth byreducing the growth rate due to precursor losses and degrade the crystalquality since the Si droplets are carried to the crystal growth surfaceby the H₂ carrier gas. Moreover, liquid aerosol particles are viscousand adhere to the gas delivery system (parasitic deposition) and causessevere degradation of the reactor parts during epitaxial growth. Thesedepositions (parasitic deposition) are generally flaky, loosely bound,and can be carried to the growth surface during the growth resulting indegradation of crystal quality, introducing defects in the growingepitaxial film. The aforesaid condition is specifically severe at higherprecursor gas flow rates required to achieve high film growth rates.Long duration epitaxial growth to achieve thick epitaxial films is alsovery inconvenient using conventional Si precursors due to excessive Sicluster formation and parasitic deposition in the reactor.

Typical growth rates using silane as the Si precursor in SiC CVD are 1μm/hour-10 μm/hour. Supersaturation and Si cluster formation prohibitincreased rate of mass transport by higher flow rate of the silaneprecursor. At increased flow, formed Si clusters degrade crystal qualityas noted earlier. Cluster formation of particles (Si) is the leadingcause of yield loss in semiconductor processing and the criticalparticle size should be reduced as the microelectronic size decreases.

The major drawback associated with the silane chemistry in achievinghigh growth rates is the relatively weak bond strength of the Si—H (318kJ/mol) bond in SiH₄ causing it to dissociate easily (and very early inthe gas delivery system) into elemental Si. In high temperature siliconcarbide (SiC) chemical vapor deposition (CVD) using silane, thedissociated elemental silicon with free dangling bonds can easily formthe Si—Si bond during their collisions and initiate liquid Si droplet oraerosol formation. The condition for Si droplet or aerosol formation isparticularly severe in SiC CVD (compared to Si CVD), using silane gas,where high temperature (typically 1550° C.) is essential to achieve SiChomoepitaxial growth.

To prevent Si droplet formation in SiC CVD, chlorinated precursors (fore.g., SiCl₄, SiHCl₃, SiH₂Cl₂) are typically used. The silicon-chlorinebond is higher in chlorosilanes (for e.g., dichlorosilane, DCS, SiH₂Cl₂)compared to silicon-hydrogen bond in silane (381 kJ/mol versus 318kJ/mol). The silicon-chlorine bond strength in dichlorosilane is strongenough to prevent silicon droplet formation in low temperature Si CVDgrowth (typically 1000° C.) where the stronger Si—Cl bond restrictselemental Si formation. However, in higher temperature SiC growth(typically 1500° C. or above), dichlorosilane (SiH₂Cl₂) can alsogenerate silicon droplets with increased parasitic deposition at highergas flow rates leading to degraded epilayer surface morphology.Parasitic deposition in the reactor, using DCS, is discussed later incomparison to conventional silane gas (see also, FIGS. 2-3).

Silicon tetrafluoride gas has been used for polycrystalline siliconcarbide films deposited by low power radio frequency plasmadecomposition of SiF₄. Silicon tetrafluoride gas has also been used forgrowing μ-crystalline 3C growth using low temperature hotwire CVD usingSi substrate. However, homoepitaxial growth on SiC substrates by hotwall, high temperature CVD has not been reported for high quality, thickfilm growth. Moreover, it is believed in the art that SiF₄ is not asuitable gas for SiC epitaxial growth due to its “too strong” Si—F bond(565 kJ/mol).

A continuing need exists for higher growth rates that can result in ahigh quality, thick homoepitaxial SiC epilayer, particularly thosemethods that can inhibit and/or prevent formation of silicon dropletsand/or parasitic growth during CVD.

SUMMARY

Objects and advantages of the invention will be set forth in part in thefollowing description, or may be obvious from the description, or may belearned through practice of the invention.

Non-destructive pretreatment methods are generally provided for asurface of a SiC substrate with substantially no degradation of surfacemorphology thereon. In one particular embodiment, a molten mixture(e.g., including near KOH-eutectic and a buffering agent) is applieddirectly onto the surface of the SiC substrate to form a treated surfacethereon. An epitaxial film (e.g., SiC) can then be grown on the treatedsurface to achieve very high (e.g., up to and including 100%) BPD to TEDconversion rate close to the epilayer/substrate interface.

In one embodiment, the buffering agent can include an alkaline earthoxide (e.g., MgO). For example, the molten mixture can include analkaline earth oxide in an amount of about 1% to about 80% by weight,such as about 5% to about 20% by weight. The molten mixture, in certainembodiments, can include KOH, the buffering agent, and at least oneother salt (e.g., NaOH, KNO₃, Na₂O₂, or a mixture thereof).

The molten mixture has, in one embodiment, a temperature of about 170°C. to about 800° C. when applied onto the SiC substrate, and can beapplied onto the surface of the SiC substrate for a treatment durationdepending on the composition of the molten mixture and the temperatureof the mixture (e.g., about 1 minute to about 60 minutes).

The epitaxial film grown on the treated surface can be a SiC film. Forexample, epitaxial growth of a SiC film can be achieved via chemicalvapor deposition utilizing a Si-source gas and a carbon-source gas. Inone particular embodiment, the SiC film can be grown via chemical vapordeposition in the presence of fluorine atoms.

In another embodiment, the molten mixture can be applied onto a bufferepilayer on the surface of the SiC substrate to form a treated surfacethereon; and the epitaxial film can then be grown on the treatedsurface. That is, prior to applying the molten mixture, the bufferepilayer can first be grown on the surface of the SiC substrate, whereinthe buffer epilayer comprises SiC.

In still another embodiment, a method of growing a bulk crystal isgenerally provided. For example, the molten mixture can be applied ontoa surface of a seed substrate to form a treated surface thereon, whereinthe seed substrate comprises SiC; and a bulk crystal can be grown on theabove treated surface, wherein the bulk crystal comprises SiC.

Other features and aspects of the present invention are discussed ingreater detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

A full and enabling disclosure of the present invention, including thebest mode thereof to one skilled in the art, is set forth moreparticularly in the remainder of the specification, which includesreference to the accompanying figures.

FIG. 1 shows a general schematic diagram of an exemplary hotwall CVDfurnace, along with a comparison of axially split gas injector tubesshowing parasitic deposition after epitaxial growth of a SiC epilayerusing silane as the Si-source gas.

FIG. 2 shows axially split gas injector tubes after use in a hotwall CVDfurnace, as diagramed in FIG. 1, comparing parasitic deposition ofdifferent Si-source gases: silane, DCS, and SiF₄.

FIG. 3 shows the mass measurement of parasitic depositions after 1 hourin a hotwall CVD furnace, as diagramed in FIG. 1, using (a) aSi-precursor gas and H₂ and b) a Si-precursor gas, propane, and H₂.

FIG. 4 a shows particles on a SiC epilayer surface grown at 7 μm/hr at 5sccm of SiH₄ (T=1550° C., P=300 torr, H₂ flow rate=6 slm).

FIG. 4 b shows particles on a SiC epilayer surface grown at 10 μm/hr at5 sccm of DCS (T=1550° C., P=300 torr, H₂ flow rate=6 slm).

FIG. 4 c shows particles on a SiC epilayer surface grown at 30 μm/hr at10 sccm of SiF₄ (T=1550° C., P=300 torr, H₂ flow rate=6 slm).

FIG. 5 shows Raman spectrum of a thick (120 μm) epilayer grown usingSiF₄ in a hotwall CVD furnace, as diagramed in FIG. 1 according toExample 1.

FIG. 6 is an X-ray rocking curve showing FWHM of 7.5 arcsec of anepilayer grown using SiF₄ according to Example 1, which indicates highquality crystal.

FIG. 7 shows a PL spectra with a 4H—SiC band edge peak at 3.17 eV of anepilayer grown using SiF₄ according to Example 1.

FIG. 8 shows microscope images of the evolution of dislocations inepitaxial growth (of sample #7 in Table 2) on the molten KOH mixturetreated (or etched) substrates according to Example 2. It shows the samearea of sample #7 on (a) the etched substrate and (b) the etchedepilayer grown on the above etched substrate. Threading screwdislocation (TSD) and threading edge dislocation (TED) on the substrate(marked S and E respectively) propagate to the epilayer. BPDs B1-B5 areconverted to TEDs, in which B2-B4 show a dislocation shift from theiroriginal positions. BPD B6 disappears. BPD B7 may propagate to theepilayer (marked N), or B7 just disappears and BPD N in the epilayer isnewly generated.

FIG. 9 shows schematically a boule (or bulk crystal) seeded from a SiCseed substrate. Also shown in the figure with dashed lines, an off-axiswafer harvested from the boule, which is used as substrate for epitaxialgrowth. The boule can be cut into on-axis (0001) or off-axis (0001)substrate wafers.

FIG. 10 shows a SiC substrate wafer off-axis cut from a boule (e.g., theboule shown in FIG. 9). When the boule is cut into off-axis (0001)substrate wafers, such as at angles of 4° or 8° that are currentlycommonly used for SiC epitaxial growth, the dislocations on the basalplane intersect the substrate surface.

FIG. 11 shows the SiC substrate of FIG. 10 after treatment by moltenKOH—NaOH—MgO mixture according to one embodiment of presently disclosed.Defect delineation or severe etching of the substrate (or visible etchpit formation) is not necessary. The size of the etch pit has noinfluence on the efficacy of BPD conversion in the subsequent epitaxialgrowth. Therefore, etch pits can be small enough that are not evenobserved by Nomarski optical microscope at ×1000 magnification, andthere is no degradation of the morphology on the entire substratesurface.

FIG. 12 shows an epitaxial film grown on the molten KOH—NaOH—MgO mixturetreated surface of the SiC substrate shown in FIG. 11. The epitaxialfilm is ready for device fabrication. Post polishing or dry etching isnot needed, and there is no limit for the thickness of the grownepilayer.

FIG. 13 shows the mechanism of BPD conversion in epitaxial growth onpure KOH and molten KOH—NaOH—MgO mixture etched surfaces. Smaller sectoropen angle ∠AOB in a BPD etch pit leads to easier pinching off of thestep-flow growth by lateral growth, resulting in a higher BPD conversionrate.

DETAILED DESCRIPTION OF INVENTION

The following description and other modifications and variations to thepresent invention may be practiced by those of ordinary skill in theart, without departing from the spirit and scope of the presentinvention. In addition, it should be understood that aspects of thevarious embodiments may be interchanged both in whole or in part.Furthermore, those of ordinary skill in the art will appreciate that thefollowing description is by way of example only, and is not intended tolimit the invention.

In the present disclosure, when a layer is being described as “on” or“over” another layer or substrate, it is to be understood that thelayers can either be directly contacting each other or have anotherlayer or feature between the layers, unless expressly stated to thecontrary. Thus, these terms are simply describing the relative positionof the layers to each other and do not necessarily mean “on top of”since the relative position above or below depends upon the orientationof the viewer or the specific application for device fabrication.

Chemical elements are discussed in the present disclosure using theircommon chemical abbreviation, such as commonly found on a periodic tableof elements. For example, hydrogen is represented by its common chemicalabbreviation H; helium is represented by its common chemicalabbreviation He; and so forth.

Various methods are generally provided for reducing and even eliminatingbasal plane dislocation density in SiC epilayers grown using hotwall CVDprocesses on a SiC substrate in order to achieve high quality epitaxy.For example, each of these process steps can be utilized alone, or incombination with each other, to achieve high quality epitaxial growth.It is noted that terms “epitaxial film” and “epilayer” are usedinterchangeably in the present disclosure.

I. Pretreatment with a Molten Mixture and Subsequent Epitaxial Growth

In one embodiment, a non-destructive pretreatment (or etch) method isgenerally provided for a SiC substrate with substantially no degradationof surface morphology thereon. After treatment (or etch), the SiCsubstrate defines a treated (or etched) surface that is particularlysuitable for subsequent epitaxial growth to achieve low or zero basalplane dislocation density in the grown epitaxial film. Thus, thissubstrate pretreatment method allows for non-destructive pretreatment ofa SiC substrate to achieve a high BPD conversion rate (e.g., up to andincluding 100%) in the subsequent epitaxial growth. As such, anepitaxial film can be grown on the surface of the treated SiC substratesuch that the BPD density on the grown epilayer is less than 10% of theoriginal BPD density on the surface of the SiC substrate. For example,in one particular embodiment, an epilayer can be grown on the surface ofthe treated SiC substrate such that the BPD density on the grownepilayer is about 0.0001% to about 1% of the original BPD density on thesurface of the SiC substrate, such as about 0.001% to about 0.5%. Incertain embodiments, the BPD density on the grown epilayer can be zero.

The molten KOH-related mixture includes, in one embodiment, KOH and abuffering agent. The buffering agent can, in one particular embodiment,be an alkaline earth oxide (such as MgO). The relative amount of thealkaline earth oxide in the molten KOH-related mixture can be anyamount, such as about 1% to about 80% by weight. For example, therelative amount of the oxide in the molten KOH-related mixture can be upto about 25% by weight, such as about 5% to about 20% by weight.

Optionally, another salt can be included in the molten mixture. Forexample, the KOH-eutectic or near eutectic can generally include KOH andat least one another salt (e.g., NaOH, KNO₃, Na₂O₂, or a mixturethereof) with any possible weight ratio, such as about 1:4 to about 4:1in terms of the weight ratio of KOH to the other salt(s) (e.g., about1:2 to about 2:1). However, the relative amounts can be varied dependingon the composition of the other salt(s) present in the molten mixture.For example, the molten mixture can, in one particular embodiment,include KOH and NaOH in a relative amount of about 1:4 to about 4:1 interms of weight ratio. Alternatively, the molten mixture can, in anotherparticular embodiment, include KOH and KNO₃ in a relative amount of 1:20to 5:1 in terms of weight ratio.

The components of the mixture (i.e., KOH, buffering agent (e.g., MgO),and one or more optional additional salt(s) (e.g., NaOH, KNO₃, Na₂O₂, ora mixture thereof)) are melted together to form a suspension mixture.For instance, in most embodiments, the buffering agent (e.g., MgO) isnot melted, but is a fine powder dispersed in molten KOH liquid to forma suspension. For example, the molten mixture may be melted attemperature of about 170° C. to about 800° C. (e.g., about 500° C. toabout 600° C.), and applied onto the surface of the substrate. Suchmelting can be carried out in any suitable container (e.g., a nickelcrucible).

The molten KOH-related mixture can be applied onto the surface of theSiC substrate (e.g., via soaking the SiC substrate in the moltenmixture) for a certain treatment duration to form a treated surfaceprior to any epitaxial growth. For example, the duration of treatmentwith the molten KOH-related mixture of the surface of the SiC substratecan be relatively short. In one embodiment, the treatment duration isabout 1 minute to about 60 minutes, such as about 2 minutes to about 5minutes. However, the treatment duration can be varied depending on thecomposition of the molten mixture and/or the temperature of the mixture.That is, the relative amounts of the each component in the moltenKOH-related mixture can be varied, while still obtaining similar resultsby adjusting treatment duration. For example, a higher KOH concentrationin the mixture and/or a higher temperature of the mixture can lead to ashorter treatment duration.

After the pretreatment, the KOH-related mixture can be removed from thesurface of the SiC substrate to reveal the treated surface thereon.Optionally, a regular RCA cleaning process can be performed on thetreated substrate prior to loading the substrate in the CVD chamber forepitaxial growth.

In one particular embodiment, the treated surface can be substantiallyfree from etch pits of dislocations thereon, while still preserving highBPD conversion rate in the subsequent epigrowth. By “substantially free”it is meant that nearly no visible pits or any other features can beseen on the treated surface using optical microscopy at ×1000magnification. As such, this treatment method can be referred to as anon-destructive treatment.

Epitaxial growth on the treated surface causes a reduced BPD density onthe subsequent epitaxial film that can be less than 10% of the originalBPD density on the substrate, while preserving the surface morphology ofthereon. For example, the epilayer BPD density can be less than 1% ofthe original BPD density on the substrate, such as less than 0.5%. Inone particular embodiment, the epilayer BPD density is less than 0.1% ofthe original BPD density on the substrate. For example, an epilayer canbe grown on the surface of the treated SiC substrate such that the BPDdensity on the grown epilayer is about 0.0001% to about 1% of theoriginal BPD density on the surface of the SiC substrate, such as about0.001% to about 0.5%. In one particular embodiment, the epitaxial filmis free of basal plane dislocations therein.

Additionally, the treated surface can have a relatively smooth surface,such as having RMS surface roughness that is less than about 1 nm, suchas less than about 0.5 nm (e.g., such as about 0.36 nm). The epitaxialfilm grown on the treated surface can also preserve this relativelysmooth surface, such as having RMS surface roughness that is less than 1nm for 8° SiC epitaxy or less than 4 nm for 4° SiC epitaxy (e.g., suchas about 0.6 nm at a 15 μm thick 8° SiC epilayer). The epitaxial filmgrown on the treated surface can be free of any pits inherited from thesubstrate treatment when a relatively short duration of treatment isemployed on the substrate. As such, this process can yield an epilayersurface that, in certain embodiments, need not be polished for furtheruse. Thus, this methodology can save valuable processing time andmaterials.

The mechanism of BPD conversion in epigrowth on the treated/etchedsubstrate is illustrated in FIG. 13. After pretreatment of the SiCsubstrate (4° or 8°) with the molten KOH-related mixture describedabove, a sector-shaped plane (marked AOB in FIG. 13) is generated at thebottom of the BPD etch pit, roughly paralleling the basal plane.Epitaxial growth of SiC on the off-axis surface is a step-flow growthmechanism. If the lateral growth (across line segments OA and OB)overcomes step-flow growth (starting from vertex O) in the etch pit(i.e. the lateral growth pinches off the step-flow growth inside theetch pit), the path for the BPD propagation in the basal plane is“blocked” and thus the BPD line will redirect perpendicularly to thebasal plane, becoming a TED. On the molten KOH-related mixture etchedsubstrate surface, the sector opening angle of BPD etch pit)(∠AOB=14°(as shown in FIG. 13 (b) on 8° SiC substrate and FIG. 13 (c) on 4°substrate) is much narrower than that on the molten pure KOH etchedsubstrate surface)(∠AOB=53° (as shown in FIG. 13 (a)). Therefore,pinching off of the step-flow growth (i.e. merging of OA and OB) bylateral growth is significantly enhanced, resulting in a very high BPDconversion rate. Furthermore, on the molten KOH-related mixture etchedbuffer epilayer surface, the sector open angle ∠AOB is only 6° (as shownin FIG. 13 (d)) in the BPD etch pit, so pinching off of the step-flowgrowth is always obtained, resulting in 100% BPD conversion.

In the present invention, formation of large etch pits in not necessary.Formation of sufficiently large BPD etch pit, as shown in FIG. 13(b)-13(d) was only for defect mapping by optical microscope to study theinfluence of etch pit size on BPD conversion and to investigate the BPDconversion mechanism. It was found that the shape of BPD etch pit isindependent of its size and hence the BPD conversion rate is notinfluenced by the size of etch pit. In the practical application,non-destructive pretreatment of the substrate is realized by ashort-time treatment, generating very small etch pits invisible byoptical microscope at ×1000 magnification. The subsequent epitaxialgrowth preserves the high BPD conversion rate due to the abovemechanism. And due to the short-time treatment, the morphology of samplesurface is not degraded.

FIGS. 9, 10, 11, and 12 sequentially show an exemplary method of forminga SiC grown boule (FIG. 9), cutting an off-axis SiC substrate wafer fromthe boule (FIG. 10), treating the SiC substrate wafer with the moltenmixture to form a treated surface (FIG. 11), and forming an epilayer onthe surface of a SiC substrate wafer (FIG. 12).

Referring to FIG. 9, a seed substrate (10) is shown having defects (12)on its surface (14). A bulk crystal (or boule) (16) of SiC is showngrown on the surface 14 of the seed substrate 10. The bulk crystal(boule) 16 has basal plane dislocations (BPDs) (18) lying in the basalplane {0001} in random orientations and threading dislocations (19)oriented in the direction of c-axis.

The SiC boule 16 can be cut to form a SiC substrate wafer configured forepitaxial growth. Generally, the boule 16 is cut at an angle (θ) fromthe basal plane that is about 1° to about 10° (e.g., about 3° to about8°). The off cut of the substrate (20) results in a surface (22) withatomic steps. During CVD growth, the adsorbed precursors migrate on thesubstrate surface and are incorporated into the epilayer at step edges.This is called “step-flow epitaxy”. Since the SiC substrate 20 is cut atthe angle (θ) from the basal plane, both the basal plane dislocations 18and the threading dislocations 19 intersect the surface 22 of the SiCsubstrate 20, as shown in FIG. 10. These dislocations can propagate intothe subsequent epilayer and hence can influence the device performance.It is noted that these dislocations are not revealed unless the surface22 is performed sufficient KOH or KOH-eutectic defect selective etching.The total number of BPDs 18 on the surface 22 of the SiC substrate 20 ina certain surface area defines the original BPD density on thesubstrate. For example, in most SiC substrates, BPDs can exist with adensity of about 10³ to about 10⁴ cm⁻².

In regular SiC epitaxial growth on the 8° untreated substrate using DCSas the Si-precursor, about 99.0% of the BPDs on the original substratesurface are converted to TEDs, resulting in about 200 cm⁻² BPDs on theepilayer surface. However, this number is still too high for commercialdevice production.

According to one particular embodiment of the present invention, thesurface 22 of the SiC substrate 20 in FIG. 10 is treated using themolten mixture as discussed above. Referring to FIG. 11, at thenon-destructive condition, the treated surface (23) has very small “etchpits” (21) (which are not observed by optical microscope) generated atthe intersecting point between dislocations 18 and 19 and surface 22.Using a scanning electron microscope (SEM) at the magnification of 100k, only a few threading screw dislocations (TSDs) with diameter lessthan 400 nm are observed on surface 23; TED and BPD etch pits 21 existbut are not observed by SEM due to the very small and shallow pits.Additionally, the treated surface 23 has no degradation of morphology onthe entire surface (such as RMS of about 0.36 nm).

Referring to FIG. 12, an epilayer 24 can be grown on the treated surface23 of the SiC substrate 20, according to any suitable method discussedin greater detail below. For example, the epilayer 24 can be grown via aCVD process using a carbon source gas (e.g., a hydrocarbon, such aspropane) as the carbon precursor and a Si-source gas (e.g., silane, achlorinated silane, (e.g., dichlorosilane), or a fluorinated silane(e.g., silicon tetrafluoride)) as the silicon precursor. Whendichlorosilane is used as the silicon precursor, in epitaxial film 24,about 99.9% of the BPDs 18 on the original substrate surface 23 wereshown to be converted to TEDs (33) during the initial stage of theepitaxial growth (close to the epilayer/substrate interface). The restsubstrate BPDs propagate into the epilayer, resulting in a BPD (34)density on the epilayer surface less than 20 cm⁻². When silicontetrafluoride was used as the silicon precursor, zero-BPD is achieved inthe epitaxial film 24 indicating that all of the BPDs 18 on the originalsubstrate surface were converted to TEDs 33.

The above methods are discussed with respect to treatment of the actualsurface of the substrate (i.e., the molten KOH-related mixture isapplied directly onto the surface of the SiC substrate). However, inanother embodiment, the molten KOH-related mixture can be applied onto abuffer epilayer that is present on the surface of the SiC substrate.That is, prior to applying the molten KOH-related mixture, the bufferepilayer (of any doping concentration; n or p type) can be grown on thesurface of the SiC substrate according to any of the methods disclosedwith respect to the “main” epilayer, since the epi-buffer layergenerally includes SiC in most embodiments.

When a buffer epilayer is present on the substrate's surface, the “main”epilayer is grown on the buffer epilayer after the pretreatment of thebuffer epilayer by the molten KOH-related mixture at the conditiondescribed above. In the presence of the buffer epilayer, zero-BPD wasobtained on the main epilayer using CVD with any combination of carbonsource gas and Si-source gas.

In all of the above methods, there is little-to-no degradation ofmorphology on the epilayer surface (35), so there is no need to performpost polishing and dry etching for the grown epilayer 24 for devicefabrication. Without wishing to be bound by any particular theory, it isbelieved that the reduction of dislocation density and/or stacking faultdensity in the grown epitaxial films can be attributed to thenon-destructive pretreatment of the SiC substrate and the use offluorinated precursor(s).

Through epitaxial growth, an epitaxial film can be grown on the treatedsurface of the SiC substrate according to known methods, including butnot limited to chemical vapor deposition (“CVD”). In one particularembodiment, homoepitaxial films can be grown as a SiC epilayer, such asusing a CVD process with a carbon-source gas (e.g., a hydrocarbon gassuch as propane) as the carbon precursor and Si-source gas (e.g., achlorinated silane such as dichlorosilane or fluorinated silane such asSilicon tetrafluoride (SiF₄)) as silicon precursor. However, in otherembodiments, heteroepitaxial films can be grown on the SiC substrate asdesired, including but not limited to graphene, AlInGaN epilayers andderivatives thereof (e.g., GaN, AlN, GaN, and combinations thereof) orto the bulk growth of SiC, GaN, AlN and derivatives thereof.

II. Silicon Carbide Homoepitaxial Growth Using Fluorine Chemistry

In one embodiment, epitaxial growth of a SiC epilayer is generallyprovided by CVD utilizing fluorine chemistry in the system. Suchepitaxial growth methods are generally provided, with or without thepretreatment of the substrate described above. Utilizing fluorinechemistry in a CVD epitaxial growth, a thick, low doped epitaxial film(commonly referred to “epilayer” or “epi”) can be grown with excellentsurface morphology, crystal quality, and polytype uniformity. Themethods of growing such epilayers utilize an environment that issubstantially free from silicon-droplets to achieve relatively highgrowth rates while maintaining the desired properties. In particular,the methods of growing the epilayers utilize fluorine in the system(e.g., SiF₄ gas as the silicon source gas) in a hot wall CVD reactor.These methods can achieve thick, low doped epilayers with excellentsurface morphology, crystal quality and polytype uniformity grown athigh growth rates in a Si droplet and Si parasitic deposition-freeenvironment, the combination of which is not attainable by using other,conventional silicon precursors. The above method of utilizing fluorinein the system (e.g., SiF₄ gas as the silicon source gas) can be easilyextended to grow bulk SiC boules of different doping concentration (n orp type) by the CVD method or in combination with the physical vaportransport (PVT) method (gas assisted PVT).

In general, fluorine in the system allows for silicon-fluorine bonds toform (or already be formed, e.g., in the case of SiF₄ gas), thusinhibiting and/or preventing silicon droplets to form in the system. TheSi—F bond strength (565 kJ/mol) is much higher compared to that of theSi—Si bond (222 kJ/mol), which is the fundamental reason for Si clustersuppression (or suppressed formation of Si—Si bonds). Additionally, theSi—F bond strength is stronger than other Si-halogen bonds (Si—Cl: 381kJ/mol; Si—Br: 309 kJ/mol; and Si—I: 234 kJ/mol). Since the Si—F bond(e.g., as in SiF₄) is the strongest of the halogens, this is the gas ofchoice to achieve the best possible condition for Si droplet-freeenvironment suitable for high temperature SiC CVD epitaxial growth overother halogenated silane gases.

Thus, growth of the epitaxial film of SiC can be achieved in the CVDchamber in an atmosphere that includes Si—Si vapor in an amount that isless than 5% by volume, due to the presence of fluorine in the systemsubstantially inhibiting and/or preventing the formation of such Si—Sibonds. For example, the CVD chamber can have an atmosphere that includesSi—Si vapor in an amount that is less than 1% by volume, and can be, inone embodiment, substantially free from Si—Si vapor. However, due to achemical reaction with the carbon source gas (and particularly ahydrocarbon gas, such as propane), an epilayer of SiC can be formed onthe substrate's surface.

By including fluorine in the system, good quality SiC epitaxial filmscan be grown at high growth rates and the doping of the grown layer canbe controlled over a wide range (e.g., semi-insulating to >10¹⁷ cm⁻³ nor p type) by adjusting the C/Si ratio or introducing nitrogen oraluminum or boron or any other appropriate dopant. Additionally,parasitic deposition in the reactor is significantly reduced whenfluorine is in the system, especially compared to conventional gases(including silane and chlorosilane gases). Reduction of silicon dropletin the chamber enables increased source gas flow rates and long termgrowth for a thicker epilayer (e.g., greater than about 100 μm,including bulk growth) with a smooth surface (e.g., a roughness RMS ofabout 0.5 nm or less).

Fluorine can be introduced into the system during growth via the use ofa fluorinated Si-source gas (e.g., SiF₄), via the use of a fluorinesource gas (e.g., HF) with any combination of a silicon and carbonsource gas or gases, or via a fluorinated carbon source gas (e.g., CF₄).All of these embodiments are discussed in greater detail below.

By using this gas chemistry (e.g., containing fluorine in the system),epi growth and the reactor environment are improved with any off cutsubstrate (e.g. 0°, 2°, 4°, 8° etc.) in any direction (e.g., 11 20, 1100etc.) or any polytype (3C, 4H, 6H etc.) or any growth planes (e.g., cplanes, m planes, a planes etc.) since a gas phase nucleation-freecondition fundamentally improves crystal growth irrespective of thesubstrate type (orientation, polytype etc.) and regardless of thediameter of the wafer. This chemistry improves epi crystal quality inany crystal growth reactor (e.g. horizontal, vertical, planetary, singleor multiple wafer reactor, etc.) since the basic principle of masstransport is applicable to any reactor geometry.

CVD growth of the epitaxial film of SiC can be achieved at a growth rateof about 1 μm/hour or faster (e.g., about 1 μm/hour to about 30 μm/hour,or about 30 μm/hour or faster) to any desired thickness (e.g., anepilayer thickness of greater than about 100 μm, from about 1 μm toabout 100 μm, or smaller than about 1 μm). In one particular embodiment,growth can be achieved in a hotwall CVD chamber at a growth temperatureof about 1400° C. to about 2000° C. (e.g., about 1500° C. to about 1800°C.). The resulting epitaxial film include SiC, and can, in oneparticular embodiment consist essentially of SiC (e.g., consist of SiC).

The method also improves crystal quality by adding the silicontetrafluoride gas partially to other precursor gases (e.g. propane,methane, silane, dichlorosilane etc.) during the growth.

Although the presently disclosed epitaxial growth utilizing fluorinechemistry can be performed without any pretreatment on the substrate (orbuffer epilayer, if present) (described in section I above), thecombination of the use of the fluoride gas chemistry during SiC epilayergrowth and the substrate pretreatment with the molten KOH-relatedmixture is extremely effective for growth of good quality SiC epitaxialfilms as a platform to grow other materials such as graphene andsemiconductors including compound semiconductors, including but notlimited to GaN, AlGaN, and InN.

Clearly, the use of fluoride gas chemistry in CVD growth is effective ingrowing good quality SiC epitaxial films at different growth rates andfilms of different thicknesses, and different doping concentrations anddoping types (selected from the group consisting of N⁺, N⁻, P⁺, P⁻ andsemi-insulating), using the principle of site competition epitaxy or byadding specific dopant species (e.g., nitrogen, aluminum, boron etc.).Pretreatment of the SiC substrate by molten KOH-related mixture etchingprior to the growth with a fluorine source present (e.g., via SiF₄)further enhances the BPD conversion, reduces stacking faults, andimproves the crystal quality further.

In general, any combination of source gases can be utilized to providefluorine atoms, silicon atoms, and carbon atoms within the CVD chamberduring epitaxial growth. In one embodiment, at least two source gasescan be introduced into the CVD chamber such that, upon decomposition atthe deposition temperature, fluorine atoms, carbon atoms, and siliconatoms are present in the CVD chamber. The relative amounts of eachcomponent can also be selectively controlled as desired, according tothe deposition conditions (e.g., temperature, flow rate, desired growthrate, etc.). The amount of fluorine atoms in the system is generallyenough to inhibit and/or prevent formation of Si—Si bonds in thedeposition conditions. Further, in another embodiment, a singleprecursor gas containing Si, C, and F, such as methyltrifluorosilane,can be used as the source gas, with or without any additional sourcegas(es) present in the chamber.

A. A Fluorinated Si-Source Gas and a Carbon-Source Gas

As stated, an epitaxial layer of SiC can be grown via CVD, in oneembodiment, utilizing a fluorinated Si-source gas in combination with acarbon-source gas. Particularly suitable fluorinated Si-source gasinclude, but are not limited to, SiH_(x)F_(y) where x=0, 1, 2, or 3; andy=4−x. For example, silicon tetrafluoride, SiF₄ (x=0), can be utilizedas the fluorinated Si-source gas.

Although any suitable gas containing carbon can be used as thecarbon-source gas, one particularly suitable class of carbon-sourcegases for this embodiment includes hydrocarbon gasses (e.g., propane,ethylene, or mixtures thereof).

The volumetic ratio of the fluorinated Si-source gas to thecarbon-source gas can vary depending on the deposition conditions in theCVD process, but is, in most embodiments generally sufficient to grow aSiC epilayer that has close to a 1:1 stoichiometric ratio of Si to C. Ingeneral the ratio of the gas flow rates (measured in standard cubiccentimeter per minute or sccm) is kept in such a way that the ratio ofthe number of C and Si atoms is about 1:1 for the growth conditionsmentioned earlier. However, good epilayers are also grown for differentC/Si ratios (e.g. from about 0.3 to about 1.6, such as 0.3, 0.6, 0.9, 1,1.2, 1.4, 1.6).

In such a system, a sufficient amount of fluorine may be present withoutany other silicon source and/or fluorine source required in the system.Thus, in one embodiment, growth can be achieved in a CVD chamber that issubstantially free from any other silicon source gas and/or any otherfluorine source gas.

However, additional source gasses (e.g., an additional Si-source gasand/or an additional C-source gas and/or an additional F-source gas) canalso be present, but is not required in this embodiment.

B. A Si-Source Gas, a F-Source Gas, and a Carbon-Source Gas

In this embodiment, a silicon source gas is used in combination with afluorine source gas (e.g., HF, F₂, or a mixture thereof) and acarbon-source gas. In this embodiment, the relative amounts of siliconand fluorine in the system (i.e., the ratio of silicon to fluorine) canbe selectively controlled as desired. However, in one embodiment, astoichiometric ratio of F:Si can be about 4:1 or higher in order toachieve the maximum suppression of Si gas phase nucleation (consideringthat Si source gas may completely decompose and form elemental Si freeradicals with 4 free bonds).

The silicon source gas can be fluorinated (e.g., as discussed above withreference to embodiment A) or can be free of fluorine, such as silane(SiH₄)). Suitable silicon source gas that are free of fluorine include,but are not limited to, hydrosilanes (e.g., comprises Si_(x)H_(y), wherex is 1, 2, 3, or 4; and y=2(x)+2), chlorinated silane gases (e.g.,dichlorosilane, trichlorosilane, tetrachlorosilane), chlorinatedcarbon-silicon source gases (e.g., methyltrichlorosilane (CH₃SiCl₃)), ormixtures thereof.

Although any suitable gas containing carbon can be used as thecarbon-source gas, one particularly suitable class of carbon-sourcegases for this embodiment includes hydrocarbon gasses (e.g., propane,ethylene, or mixtures thereof).

Additional source gases (e.g., an additional Si-source gas and/or anadditional C-source gas and/or an additional F-source gas) can also bepresent, but is not required in this embodiment.

C. A Combination Si- and C-Source Gas, and a F-Source Gas

In this embodiment, a combination Si- and C-source gas is used incombination with a fluorine source gas (e.g., HF). In this embodiment,the relative amounts of silicon and fluorine in the system (i.e., theratio of silicon to fluorine) can be selectively controlled as desired.

The Si- and C-source gas can be fluorinated (e.g.,methyltrifluorosilane), or can be free of fluorine, such asmethyltrichlorosilane (CH₃SiCl₃).

Additional source gases (e.g., an additional Si-source gas and/or anadditional C-source gas and/or an additional F-source gas) can also bepresent, but is not required in this embodiment.

D. A Fluorinated Carbon Source Gas and a Si-Source Gas

In this embodiment, a fluorinated C-source gas is used in combinationwith a Si-source gas. Particularly suitable fluorinated C-source gasinclude, but are not limited to, CH_(x)F_(y) where x=0, 1, 2, or 3; andy=4−x. For example, tetrafluoromethane, CF₄ (x=0), and/ortrifluoromethane, CF₃H (x=1), can be utilized as the fluorinatedC-source gas.

The silicon source gas can be fluorinated (e.g., as discussed above withreference to embodiment A) or can be free of fluorine, such as silane(SiH₄)). Suitable silicon source gas that are free of fluorine include,but are not limited to, hydrosilanes (e.g., comprises Si_(x)H_(y), wherex is 1, 2, 3, or 4; and y=2(x)+2), chlorinated silane gases (e.g.,dichlorosilane, trichlorosilane, tetrachlorosilane), chlorinatedcarbon-silicon source gases (e.g., methyltrichlorosilane (CH₃SiCl₃)), ormixtures thereof.

Additional source gases (e.g., an additional Si-source gas and/or anadditional C-source gas and/or an additional F-source gas) can also bepresent, but is not required in this embodiment.

Example 1 Fluorine Chemistry

Experiments were conducted in a hot wall CVD reactor. Silicontetrafluoride (SiF₄) was used as the gas precursor for silicon source aswell as conventional gases (silane and dichlorosilane) for comparisons.Propane gas was used as the carbon source, whereas hydrogen gas was usedas the carrier gas. Growth temperature was kept at 1550° C. and thereactor pressure was kept fixed at 300 Torr. The C/Si ratio wasmaintained at 1. Commercially available 4H—SiC (Si face, 8° or 4° offcut towards [1120] direction) substrates were used, without any surfacepretreatment.

A novel gas delivery tube system (gas injector) was used to visualizethe parasitic deposition in the gas delivery tube. This design was aneffective tool to identify the location at which gases start decomposingin the injector tube by the observation of parasitic depositions. Inthis scheme, the gas delivery tube is axially split into two halves,which can be assembled together for epitaxial growth. The scheme isshown in FIG. 1 where the split part of the gas delivery tube can beseen as PQ in the CVD reactor. These split halves are assembled togetherto form a complete tube before growth and can be separated again afterthe growth for observation. Inside the injector tube, a temperaturegradient was formed, with two temperature points shown in FIG. 2, withthe susceptor (i.e., the wafer holder) shown in FIG. 1 as the box belowthe end of the injector tube (Q) heated to 1550° C.

The inside image of one half of the split tube before and after thegrowth is shown in FIG. 1. Here it can be seen that the tube is cleanbefore the growth. However, after the growth, parasitic deposition,which was a composition of different Si and C compounds, can be clearlyseen. The gas decomposition condition in the tube can be roughlyestimated from the locations of parasitic deposition regions in thetubes. This technique was specially proven to be beneficial forcomparing different gas chemistries in a CVD reactor (FIG. 2). Thisprocess was very useful to visualize the gas decomposition condition inthe reactor for growth optimization.

The parasitic deposition and gas decomposition using SiF₄, in comparisonto conventional gases, was demonstrated in FIG. 2 using the aforesaidsplit tube scheme of FIG. 1. In FIG. 2, three split gas injector tubesare shown after the epitaxial growths with different gases using samegrowth conditions. A gas mixture of the silicon precursor gas(SiH₄/SiCl₂H₂/SiF₄), propane and hydrogen, entered the cold end of thetube (right) and exited to the hot end (left) towards the substrate.During this travel, gas decomposition took place and material depositedon the tube wall from the decomposed gas. It can be clearly seen in FIG.2 that silicon tetrafluoride (SiF₄) decomposed much later in the tubeand produced the least amount of deposition on the tube wall compared tosilane (SiH₄) and dichlorosilane (SiCl₂H₂). The measured weights ofthese depositions were shown in a bar graph in FIG. 3.

Approximately ˜0 mg of Si deposition (compared to 341 mg for SiH₄ and235 mg for DCS) was measured for the case of SiF₄ without propaneindicating Si deposition-free condition in the reactor forSiF₄—essential for high quality growth in a clean reactor environment(implying minimum Si pyrolysis and minimum gas phase nucleation). On theother hand, with propane addition, only 71 mg of parasitic depositiontook place on the gas injector tube for SiF₄ compared to 370 mg and 323mg respectively for SiH₄ and DCS (no significant difference was observedfor DCS and SiH₄ gases). This suppression of parasitic deposition andgas phase nucleation using SiF₄ not only improved the crystal quality byminimizing Si and SiC parasitic particles originating from the reactorparts but also increased the re-usability of the reactor parts, which isan important factor to reduce the growth cost. This is also an importantconsideration for long duration growth.

A significant reduction of parasitic deposition using SiF₄ wasindicative of a reduced gas phase nucleation condition considering thata reduced gas phase reaction (molecule-molecule interaction) was also acondition for reduced parasitic deposition (molecule-solid interaction)and vice versa. Thus, it is believed that the use of SiF₄ achieves a Sidroplet-free environment, which is not possible to achieve usingconventional gases (e.g., silane, chlorinated silicon precursors, etc.)under the same conditions.

TABLE 1 Comparison of epilayer quality using various precursors for (T =1550° C., P = 300 torr, H₂ flow rate = 6 slm, C/Si = ~1 and growthduration = 1 hr; substrate E₂(TO)/E₁(TO) or 4H/3C peak ratio = ~32,substrate doping = ~1 × 10¹⁹ cm⁻³ N-type and substrate XRD FWHM = ~20arcsec). Raman Surface Particle Particle Particle Si gas Growth E₂ to E₁roughness density density density Pit density Si flow rate rate (4H/3C)Doping R.M.S. XRD 100-400 μm 30-100 μm 10-30 μm 1 μm-3 μm precursor(sccm) [a] (μm/hr) Peak ratio (cm⁻³) (nm) [b] FWHM (cm⁻²) (cm⁻²) (cm⁻²)(cm⁻²) SiH₄ 5 7 ~30 1 × 10¹⁷ p ~0.5 nm ~20 20 ~50 ~200 ~3 × 10⁵ DCS 5 10~50 5 × 10¹⁶ p ~0.4 nm ~12 0 ~45 ~100 ~2 × 10³ SiF₄ 10 30 ~60 1 × 10¹⁵ n~0.3 nm ~7.5 0 0 ~5 ~50 [a] epilayer surface significantly degrades withparticulates at these flow rates (5 sccm) for silane and DCS. However,growth is improved due to suppression of particulate using SiF₄ even forhigher flow rates (10 sccm). FIGS. 4a, 4b, and 4c show the growthsurfaces formed at these flow rates using SiH₄, DCS, and SiF₄,respectively. [b] excluding the particulates in the epilayers.

A large number of particle related defects were observed for the growthusing SiH₄ at 5 sccm for one hour growth at a growth rate of ˜7 μm/hr,as shown in FIG. 4 a. These large particles were directly related to thedegradation of the reactor parts due to heavily deposited Si and theirconsequent exfoliation. Long duration growth with good morphology wasnot possible in this case. The density of particle related defects isshown in Table 1. Growth using DCS exhibited somewhat lower density ofparticles (due to less parasitic deposition). Long duration growth withgood quality epilayers was still not possible even at this growthcondition due to these particles. On the other hand SiF₄ significantlysuppressed gas-phase nucleation and parasitic deposition at the sameconditions, and a significantly higher growth rate was achieved byincreasing the mass transport without generating particles. In thiscleaner growth environment, long duration growth with good qualityepitaxy is possible and films greater than 120 μm thicknesses weregrown. A comparison of particulates generated during growth using threedifferent Si precursors is shown in Table 1.

Homoepitaxial films were grown using silicon tetrafluoride (SiF₄) forthe first time in a Si droplet-free condition. Epitaxial films with 10μm, 30 μm, 60 μm and 120 μm thicknesses were grown. Very smooth surface(RMS roughness ˜0.3 nm) was observed even for a ˜60 μm thick epi grownat 30 μm/hr. The surface roughness did not increase significantly forthicker (˜120 μm), long grown (4 hours) epilayers (RMS roughness ˜0.5 nmvs. ˜0.3 nm).

As shown in FIG. 5, Raman spectra of a 120 μm thick Epilayer wasexamined and showed that the ratio between 4H peak (at wave number 776)and 3C peak (at wave number 796) as an indication of the polytypeuniformity for the case where the epitaxial film was grown on a 4Hsubstrate. Negligible height of 3C peak compared to the 4H peak heightdemonstrated very low formation of 3C polytype in the epilayer. A peakratio (4H/3C) of ˜60 (Table1) indicated excellent polytype uniformity(4H) for the growth using SiF₄.

X-ray rocking curve was obtained to measure the crystalline quality ofthe epi grown using SiF₄. A FWHM of a 60 um thick sample was found to be˜7.5 arcsec (FIG. 6), which indicated very high crystal quality.

The room temperature photoluminescence (PL) spectra (FIG. 7) showed thetypical exciton/band edge peak for 4H SiC at 3.17 eV (391 nm),indicating no change of bandgap of the SiC epilayer grown using fluorinechemistry. Molten KOH etching of the epilayer demonstrated very lowdensity of BPDs. For an 8° off cut substrate a density of ˜20 cm⁻² wasobserved, and for a 4 degree off cut substrate a density of ˜5 cm² wasobserved.

Example 2 Substrate Surface Pretreatment

The substrate was commercially obtained 4H—SiC wafer with 8° off-axistowards [11 20] direction with the Si-face chemical mechanical polished.The substrates were treated by molten KOH-related mixture of (e.g.,KOH—NaOH—MgO at 35:50:15 wt %) for different durations (2-45 min).Epitaxial growth was carried out in a home-built chimney CVD reactor at1550° C. and 80 Torr, using propane and dichlorosilane as precursors.The doping of the epilayer is ˜1×10¹⁵ cm⁻³ n-type was controlled by C/Siratio. After growth, all the samples were etched by the same etchant todelineate the defects on the epilayers.

TABLE 2 Sample pretreatment condition and defect density on theepilayers grown on 8° SiC substrates ^(a). Substrate Etch pit EpilayerBPD density Sample treatment size on the thickness on the epilayer No.duration substrate (μm) (cm⁻²) 1   No — 6 183 2   No — 6 150 3   No — 6232 4    2 min Invisible ^(b) 6 20 5   12 min   <1 μm 6 11 6   25 min~1.5 μm 6 22 7   45 min   ~4 μm 6 15 8 ^(c)  2 min Invisible ^(b) 0.5 19^(a) Typical BPD density on 8° SiC substrate is 2.8 × 10⁴ cm⁻². ^(b) Theetch pits are “invisible” under a Nomarski optical microscope at themagnification of ×1000. ^(c) Sample #8 was performed reactive ionetching (RIE) leaving a thickness of 0.5 μm epilayer before final defectdelineation.

Table 2 shows the treatment condition of the SiC substrates and defectdensity in the corresponding epilayers. The typical BPD density on thesubstrate surface is ˜2.8×10⁴ cm⁻². For regular growth on the untreatedsubstrate (Samples #1˜#3), the BPD density on the epilayer was 150˜232cm⁻². When the substrate was pre-treated by the molten mixture for 2˜45min (Samples #4˜#7), the BPD density reduced to 11˜22 cm⁻² on theepilayer, which is <0.1% of BPDs on the substrate. The reduction of BPDwas independent of the treatment duration (i.e., etch pit size generatedon the substrate surface). Shorter time treatment, such as a few minutessoaking in the mixture, did not form any visible etch pits wheninspected by Nomarski optical microscope at ×1000 magnification; andthere was no surface degradation anywhere on the entire sample surface.Therefore, a few minutes (2 min by example) soaking (pretreatment) inthe molten KOH-related mixture can be regarded as a non-destructivesubstrate pretreatment method to achieve significant reduction of BPDs.The AFM roughness of sample #4 was compared to the epilayer roughness ofsample #1 (grown on the untreated substrate). It is found that after 2min treatment, the substrate surface of sample #4 did not show anyincrease of roughness. After epitaxial growth, the epilayer roughness ofsample #4 (RMS=0.54 nm) was comparable with that of sample #1 (RMS=0.59nm) which was grown on the untreated substrate.

The BPD evolution in the above process is investigated from sample #7which substrate was etched for 45 min by the molten KOH-related mixtureresulting in etch pits large enough for defect mapping. FIG. 8 showsmicroscope images of the same area on the etched substrate and on theetched epilayer grown on the same substrate. It is found that most(about 99.9%) BPDs on the substrate were converted to TEDs (BPDs B1˜B5in FIG. 8) or disappear (BPD B6 in FIG. 8). Very few (about 0.1%) BPDson the substrate propagate into the epilayer (BPD B7 in FIG. 8). Furtheroptimization of the temperature profile in the furnace and decompositionprocess of precursor gases can completely eliminate these BPDs toachieve a BPD-free epilayer (100% conversion of BPDs on the substratesurface).

In order to know where the BPDs were converted in the epilayer, sample#8 substrate was given a 2 min pretreatment prior to epigrowth. Thegrown epilayer (˜6 μm) was then etched by reactive ion etching (RIE) toremove most part of the epilayer, leaving an epilayer of ˜0.5 μmthickness. Then, it was etched to reveal defects on the thin episurface. Table 2 shows that the BPD and TED densities on this 0.5 μmthickness epilayer were similar to those on the thicker (˜6 μm) epilayer(samples #4-#7). This implied that all the BPD conversions occurredwithin the 0.5 μm thickness, i.e., very close to the epilayer/substrateinterface.

The molten mixture was also employed to pretreat the 4° off 4H—SiCsubstrates or buffer epilayers thereon prior to epitaxial growth. Asshown in Table 3, for epitaxial growth on 4° SiC substrate (whethertreated or untreated), the BPD density on the epilayer decreased withincreasing epilayer thickness, indicating that BPDs were convertedthroughout the epilayer thickness. For growth on the pretreatedsubstrates, the BPD density decreased from 12 to 2.5 cm⁻² as epilayerthickness increased from 1.5 to 15.2 μm, which is approximately oneorder of magnitude less than that in epilayers grown on untreatedsubstrates. Therefore, the pretreatment method greatly enhanced the BPDconversion in the vicinity of epilayer/substrate interface. A 3 min (orless) soaking of the 4° off SiC substrate in the molten KOH-relatedmixture can be applied as a standard non-destructive pretreatment methodpreserving the high BPD conversion rate. For epitaxial growth on thebuffer-epilayer which is pretreated by the molten mixture, zero-BPD wasachieved in the subsequently grown main epilayers even for very thinlayers (as shown in samples #23-25 in Table 3).

TABLE 3 Pretreatment duration of 4° SiC substrate ^(a) (or 1.5 μm thickbuffer epilayer for samples 23-25) by the molten KOH-related mixture andBPD density on the epilayer (or on the main epilayer for samples 23-25)for different epilayer thicknesses. Substrate (or buffer epilayer)Epilayer BPD density Sample pretreatment thickness on the epilayer No.duration (μm) (cm⁻²) 9 No 1.5 85 10 No 3.5 62 11 No 6.9 41 12 No 8.1 6213  3 min 1.5 12 14  3 min 3.6 9.1 15  3 min 3.6 4.0 16  3 min 6.1 2.617  3 min 7.2 7.5 18  3 min 7.2 4.5 19 15 min 7.2 7.1 20 40 min 7.2 5.221  3 min 15.2 4.7 22  3 min 15.2 2.5 23  2 min 1.5 0 24  2 min 3.5 0 25 2 min 6.5 0 The average BPD density on the 4° SiC substrate is 8.4 ×10³ cm⁻².

In summary, treating the SiC substrate with the molten mixture for a fewminutes is simple, non-destructive, and highly efficient to reduce BPDsin the subsequently grown epitaxial film. No subsequent polishing of thegrown epilayer is needed for further device fabrication. Thus, thispretreatment method shows high potential to be applied as one of theroutine treatment steps prior to SiC epitaxial growth to increase theBPD conversion.

Example 3

A fluorinated silicon precursor, silicon tetrafluoride (SiF₄), wasemployed in SiC CVD growth to investigate the pretreatment method. Thesubstrates are cut from a 4° off-axis 4H—SiC wafer and epitaxial growth(˜4 μm) is first carried out without any pretreatment and a surprisinglylow BPD density was observed (5 cm⁻²) compared to the other growthmethod mentioned in Table 2 (sample 1, 2 and 3) using DCS as the Siprecursor. Later in other experiments the surface was treated by themolten KOH-related mixture for 2 min. Epitaxial growth was then carriedout using the novel SiF₄ precursor described earlier on the pretreatedsubstrate. The grown epilayer was completely BPD free, indicating that100% BPD conversion was achieved (Table 4) at the initial stage ofepigrowth (i.e., very close to the epilayer/substrate interface). Thisaccomplishment of 100% substrate BPD conversion was accomplished fromthe substrate pretreatment combined with the use of the SiF₄ precursor.

TABLE 4 Comparison of BPD conversion using SiF₄ mediated growth with orwithout substrate pretreatment by molten KOH-related mixture etching.Epilayer grown using Epilayer grown using SiF₄ without SiF₄ withKOH-related any pretreatment mixture pretreatment BPD density (cm⁻²) 5 0

These and other modifications and variations to the present inventionmay be practiced by those of ordinary skill in the art, withoutdeparting from the spirit and scope of the present invention, which ismore particularly set forth in the appended claims. In addition, itshould be understood that the aspects of the various embodiments may beinterchanged both in whole or in part. Furthermore, those of ordinaryskill in the art will appreciate that the foregoing description is byway of example only, and is not intended to limit the invention sofurther described in the appended claims.

1. A method of converting basal plane dislocations on a surface of a SiCsubstrate to threading edge dislocations, the method comprising:immersing the SiC substrate into a suspension mixture having atemperature of about 170° C. to about 800° C., wherein the suspensionmixture comprises KOH and an alkaline earth oxide; and thereafter,growing an epitaxial film on the surface of the SiC substrate.
 2. Themethod as in claim 1, wherein the SiC substrate is a SiC wafer, andwherein the surface is defined by the SiC wafer.
 3. The method as inclaim 1, wherein the SiC substrate comprises a SiC wafer having a bufferepilayer thereon, and wherein the surface of the SiC substrate isdefined by the buffer epilayer.
 4. The method as in claim 3, wherein thebuffer epilayer comprises SiC.
 5. The method as in claim 3, furthercomprising: prior to immersing the SiC substrate into the suspensionmixture, growing the buffer epilayer on the surface of the SiC wafer. 6.The method as in claim 1, wherein the suspension mixture comprises thealkaline earth oxide in an amount of about 5% to about 80% by weight. 7.The method as in claim 6, wherein the suspension mixture comprises thealkaline earth oxide in an amount of about 5% to about 20% by weight. 8.The method as in claim 1, wherein the suspension mixture comprises KOH,the alkaline earth oxide, and at least one other salt.
 9. The method asin claim 8, wherein the at least one other salt comprises NaOH, KNO₃,Na₂O₂, or a mixture thereof.
 10. The method as in claim 1, wherein thesuspension mixture further comprises NaOH.
 11. The method as in claim10, wherein the suspension mixture comprises KOH and NaOH in a relativeamount of about 1:4 to about 4:1 in terms of weight ratio.
 12. Themethod as in claim 1, wherein the suspension mixture further comprisesKNO₃.
 13. The method as in claim 12, wherein the suspension mixturecomprises KOH and KNO₃ in a relative amount of 1:20 to 5:1 in terms ofweight ratio.
 14. The method as in claim 1, wherein the suspensionmixture further comprises Na₂O₂.
 15. The method as in claim 1, whereinthe SiC substrate is immersed within the suspension mixture for atreatment duration that is about 1 minute to about 60 minutes.
 16. Themethod as in claim 1, wherein the epitaxial film grown on the treatedsurface comprises SiC.
 17. The method as in claim 16, wherein theepitaxial growth is achieved via chemical vapor deposition utilizing aSi-source gas and a carbon-source gas.
 18. The method as in claim 17,wherein the epitaxial growth is performed in the presence of fluorine.19. The method as in claim 1, wherein the surface of the SiC substratehas an original BPD density prior to immersing into the suspensionmixture, and wherein the epitaxial film is grown on the surface of theSiC substrate such that the epitaxial film has a BPD density that isabout 0.0001% to about 1% of the original BPD density on the surface ofthe SiC substrate.
 20. The method as in claim 1, wherein the epitaxialfilm is completely free from basal plane dislocations.
 21. A method ofconverting basal plane dislocations on a surface of a SiC seed substrateto threading edge dislocations, the method comprising: immersing the SiCseed substrate into a suspension mixture having a temperature of about170° C. to about 800° C., wherein the suspension mixture comprises KOHand an alkaline earth oxide; and thereafter, growing an SiC bulk crystalon the surface of the SiC seed substrate.
 22. The method as in claim 21,wherein the suspension mixture comprises KOH, the alkaline earth oxide,and at least one other salt.
 23. A substrate, comprising: a SiCsubstrate defining a surface; and an epitaxial film on the surface ofthe SiC substrate, wherein the epitaxial film comprises SiC and iscompletely free from basal plane dislocations.
 24. The substrate as inclaim 23, wherein the SiC substrate is a SiC wafer, and wherein thesurface is defined by the SiC wafer.
 25. The substrate as in claim 23,wherein the SiC substrate comprises a SiC wafer having a buffer epilayerthereon, and wherein the surface of the SiC substrate is defined by thebuffer epilayer, and further wherein the buffer epilayer comprises SiC.